Electric lamp with absorbing and interference media

ABSTRACT

An electric lamp includes a lamp vessel which is transparent to visible light and accommodates a light source. The lamp vessel is covered with a combination of a light-absorbing medium and an optical interference film having layers of alternately layers of silicon dioxide and layers of a material with a comparatively high refractive index (e.g. ZrO 2 ) . The reflection R of the interference film changes from 0.60≦R≦0.95 to 0.40≦R≦0.65 with a step ΔR in the range 0.2≦ΔR≦0.45, in a wavelength range 0.95×λ tr ≦λ≦1.2×λ tr . In operation, the electric lamp emits colored light in the transmission mode and has a substantially color-neutral appearance in the off state.

The invention relates to an electric lamp comprising alight-transmitting lamp vessel in which a light source is arranged,

said electric lamp comprising a light-absorbing medium exhibiting aspectral transition at a wavelength λ_(tr) in the visible range,

at least a part of the lamp vessel being provided with an interferencefilm.

Such lamps are used in automotive applications, for example, as a(halogen) headlamp which, in operation, emits yellow light, as anamber-colored light source in indicators (also referred to as vehiclesignal lamps) or as a red-colored light source in brake lights. Suchelectric lamps are also used for general illumination purposes. Saidelectric lamps are further used in traffic and direction signs, contourillumination, traffic lights, projection illumination and fiber opticsillumination. Alternative embodiments of such lamps comprise lampswherein the color temperature is increased by means of a suitablecombination of a light-absorbing coating and an interference film.

An electric lamp of the type mentioned in the opening paragraph is knownfrom the unpublished EP patent application 00204105.1 (PHNL000646). Theknown electric lamp comprises a light-absorbing medium exhibiting aspectral transition in the visible range in combination with acolor-neutral interference film with a relatively high reflection overthe larger part of the visible spectrum. The known electric lamp emitscolored light in the transmission mode and has a substantiallycolor-neutral appearance in the off state.

A drawback of the known lamp is that the luminous output of the electriclamp is relatively low.

The invention has for its object to provide a means for counteractingthe above disadvantage. According to the invention, an electric lamp ofthe kind mentioned in the opening paragraph is for this purposecharacterized in that

in a wavelength range 0.95×λ_(tr)≦λ≦1.2×λ_(tr), the reflection R of theinterference film changes from 0.60≦R≦0.95 to 0.40≦R≦0.65 with a step ΔRin the range 0.2≦ΔR≦0.45.

By reducing the reflection level of the interference film to a(slightly) lower level in the region around and above the spectraltransition at a wavelength λ_(tr), the luminous output of the electriclamp is improved while substantially retaining the color-neutralappearance of the electric lamp in the off state. The location in thevisible range of the spectral transition of the light-absorbing mediumdefines the color of the light output of the electrical lamp (e.g. foramber-colored light λ_(tr)˜550 nm and for red-colored light λ_(tr)˜600nm). In the wavelength range λ≦λ_(tr) the light-absorbing medium absorbspart of the visible light. The reflection R of the interference film isrelatively high in this wavelength range (0.60≦R≦0.95 forλ<approximately λ_(tr)). A substantial part of the light transmitted bythe light-absorbing medium is reflected back by the interference film inthe direction of the electric light source and passes thelight-absorbing medium again. The light-absorbing medium absorbs part ofthis reflected light. Part of the remainder of the non-absorbed light isabsorbed by the light-absorbing medium or reflected again by theinterference film. The light originating from the light source thuspasses through the absorbing layer more than once and also is reflecteda number of times by the interference film. As a consequence of thisprocess of multiple absorption and reflections, the thickness of theabsorbing layer can be relatively thin as compared to an electric lampcomprising an absorbing film in the absence of a reflective interferencefilm.

In the known electric lamp, the light is also reflected by theinterference film in the wavelength range where the light-absorbingmedium is no longer active (i.e. in the wavelength range λ>λ_(tr)). Inthis wavelength range, the light-absorbing medium no longer(substantially) absorbs. Due to the multiple reflections of light by theinterference film in this wavelength range, part of the reflected lightis nevertheless lost due to absorption phenomena in the electric lamp.For instance, light is absorbed and/or lost in the base or in otherparts of the lamp (e.g. the light source). Such undesired effects giverise to considerable losses in the light output of the known electriclamp. Another drawback is that the undesired multiple reflections of theemitted light induce the appearance of multiple images of the lightsource (e.g. the filament) and/or a relatively high haze level ascompared to lamps comprising an absorbing layer only.

According to the invention, the reflection level of the interferencefilm is relatively high in the wavelength range where thelight-absorbing medium is active (0.60≦R≦0.95 for λ<λ_(tr)) whereas thereflection level of the interference film is at a (slightly) lower levelin the region (around and) above the spectral transition of thelight-absorbing medium where the activity of the light-absorbing medium(gradually) diminishes (0.40≦R≦0.65 for λ>λ_(tr)). The step ΔR betweenthe reflection levels where the reflection is relatively high and wherethe light-absorbing medium is no longer active is in the range0.2≦ΔR≦0.45.

In the wavelength range above the spectral transition of thelight-absorbing medium (λ>λ_(tr)), the reflectivity of the interferencefilm is greatly diminished because of the relatively low reflectionlevel of the interference film. In addition, the reduction of theeffective number of reflections at the lower reflectivity leads to adecreased scattering contribution. A further advantage is that undesiredabsorption phenomena are also at a lower level.

It has been suggested to apply an interference film with a sharptransition (a so-called step-filter) at the spectral transition of thelight-absorbing medium. The term “step filter” is to be understood tomean that the reflection spectrum exhibits a comparatively sharpspectral transition (from R≈100% to R≦10%) in a comparatively narrowwavelength range (≦20 nm). The positioning of the spectral transition ofthis step filter is very sensitive to process variations. Smallvariations readily lead to a shift of the spectral transition, as aresult of which the electric lamp no longer meets legal requirements. Insuch an electric lamp it is also necessary to apply an interference filmhaving a comparatively high reflection in the relevant spectral range,thus necessitating a stack of a comparatively large number of opticallayers. Such high reflection values are necessary to sufficientlyenhance the comparatively small effect of the comparatively thinlight-absorbing medium. In addition, the optical layers in theinterference film of such an electric lamp must be at leastsubstantially absorption-free in order to realize the high reflectionvalues of the step filter. Additional drawbacks of this design are colorvariations as a function of the position of the electric lamp and thecolored appearance of the electric lamp in the off-state which is due tothe “zero” reflectivity (R≦10%) in the part of the spectrum where thelight-absorbing medium is active.

The interference film according to the invention can be realized with arelatively simple filter design with a relatively small number ofoptical layers. In addition, the position of the jump in reflection isnot very critical.

Preferably, in the wavelength range 0.95×λ_(tr)≦λ≦1.1×λ_(tr), thereflection R changes from 0.70≦R≦0.80 to 0.45≦R≦0.55 with a step ΔR inthe range 0.2≦ΔR≦0.3. A very suitable interference film has a reflectionlevel of approximately 0.75 in the wavelength range where thelight-absorbing medium is active (λ<λ_(tr)) and a reflection level ofapproximately 0.5 in the wavelength range where the light-absorbingmedium is no longer active (λ>λ_(tr)).

To explain the effect of the invention the situation is considered for anon-absorbing electric lamp provided with an interference film withmultiple reflections. The fraction of the emitted light as a function ofthe number of reflections can be easily calculated. For m reflections ofthe light, the amount of light T transmitted by the interference film asa function of the level of reflection R of the interference film isgiven by the following formula, which is known to the skilled person:T=1−R ^(m+1)Table I shows the total fraction of emitted light after m reflections asa function of the reflection level of the interference film according tothe above-mentioned formula.

TABLE I Total fraction of emitted light after m reflections as afunction of the reflection level of an interference film Total fractionof emitted light after m Number of reflections reflections R = 0.75 R =0.5 R = 0.4 0 0.25 0.50 0.60 1 0.44 0.75 0.84 2 0.58 0.875 0.94 3 0.680.94 0.975 4 0.76 0.97 0.99From Table I shows that for a traditional R=0.75 interference film afterfour reflections still only 76% of the light leaves the electric lampwhile the same is true for an interference film with R=0.5 after onlyone reflection. This difference is employed advantageously in thepresent invention in that the mirroring effect of the interference filmis only present in the wavelength range where the light-absorbing mediumis active.

Assuming an absorption A somewhere in the electric lamp in combinationwith an interference film, the following formula can be used tocalculate the transmitted light:

$T = \frac{\left( {1 - A} \right)\left( {1 - R} \right)}{1 - {R\left( {1 - A} \right)}^{2}}$Assuming an effective absorption A of 4% in combination with an R=0.75interference film, the transmission is approximately 78% as compared tothe absorption free situation. Reducing the reflectivity to R=0.50 leadsto a transmission of approximately 89%. According to this model thelumen loss decreases from 20% to approximately 10%.

Preferably, the interference film has a metallic, silvery or grayishappearance. An electric lamp provided with such an interference film canvery suitably be used as an indicator lamp for automotive applications.Statutory regulations define a range, in the 1931 C.I.E. color triangleknown to those skilled in the art, for the color point of the lightemitted by such indicator lamps. A suitable combination of alight-absorbing medium and an interference film applied to an outsidesurface of the lamp vessel enables the appearance of the electric lampto be changed. This particularly enables a distinction to be madebetween the appearance of the electric lamp in the off state and thecolor of the light emitted by the electric lamp during operation. Theaim is, in particular, to provide an electric lamp which, in operation,emits a certain color, for example, a so-called amber-colored orred-colored electric lamp while, in the off state, the electric lamp hasan at least substantially color-neutral appearance.

In vehicles it is desirable, for esthetical reasons, to provideindicator lamps and brake lights with a color-neutral appearance. Onlywhen the electric lamp is activated, it shows the desired color, wherebythe color point of the light emitted by the electric lamp meetsstatutory regulations. Moreover, in vehicles there is a tendency toaccommodate amber-colored indicator lamps in the same reflector as theheadlamp instead of in a separate reflector. In addition, the aim is touse luminaires in vehicles, which are provided with so-called “clearcovers”, i.e. an observer situated outside the vehicle can directly seethe indicator lamps or brake lamps in the luminaire. For reasons ofsafety, it is important that, apart from a color-neutral appearance,such indicator lamps are at least substantially free of coloring inreflection at light which is (accidentally) incident on the electriclamp. If, for example, sunlight or light originating from on-comingtraffic is incident on a headlamp of a vehicle comprising an indicatorlamp, the appearance of said headlamp, in reflection, should be at leastsubstantially colorless or, in reflection, said lamp should emit atleast substantially no color. Otherwise, this might confuse other roadusers and give rise to unsafe and/or undesirable situations.

In reflection, the spectral characteristic of the electric lamp inaccordance with the invention differs from the spectral characteristicin transmission. In transmission, the light emitted by the electric lampmeets statutory regulations with respect to the color point, while, inreflection the electric lamp is color-neutral, the appearance of theelectric lamp being, for example, silvery or grayish. The currentinvention applies, in particular, to indicator lamps and brake lights ofvehicles.

A synergetic effect is achieved using an electric lamp comprising acombination of a light-absorbing medium and an interference film givingthe electric lamp a color-neutral appearance. In addition, the presenceof the interference film may increase the stability of thelight-absorbing medium in that the interference film serves as an oxygenbarrier for the light-absorbing medium. Moreover, the interference filmcan counteract loss of color of the light-absorbing medium under theinfluence of external UV light, for example, by a suitable materialchoice, a suitably chosen band gap (for example TiO₂) or as a result ofthe fact that the interference film also reflects UV light. Experimentshave shown that the adhesion of the combination of light-absorbingmedium and interference film on the lamp vessel of the electric lamp issatisfactory and not, or hardly, subject to change during the servicelife. During the service life of the electric lamp in accordance withthe invention, no visible delamination of the applied coatings isdetected.

A further advantage of the application of an electric lamp comprising acombination of a light-absorbing medium and an interference film, givingthe electric lamp a color-neutral appearance, is that the spectralcharacteristic of the light-absorbing layer is less sensitive tovariations in the location of the spectral transition in thelight-absorbing layer. This implies that the spectral characteristic ofthe light-absorbing layer is less sensitive to variations in thethickness and/or the concentration of the light-absorbing medium.

An embodiment of an electric lamp in accordance with the invention ischaracterized in that a wall of the lamp vessel comprises thelight-absorbing medium. Light-absorbing media can be readilyincorporated in the wall of the lamp vessel, which is made, for example,from glass, such as quartz glass or hard glass, or from a translucentceramic material. In this embodiment, the interference film ispreferably directly applied to a side of the wall of the lamp vesselfacing away from the light source. As the light-absorbing medium isprovided in the wall of the lamp vessel and the interference film, lightreflected by the interference film will pass the light-absorbing.mediumtwice, which leads to a further improvement of the effectiveness of theabsorption process. In addition, light which is reflected to and frombetween the interference film on both sides of the lamp vessel passesthe light-absorbing medium twice at each reflection.

An alternative embodiment of the electric lamp in accordance with theinvention is characterized in that the light-absorbing medium comprisesa light-absorbing layer which is situated between the lamp vessel andthe interference film. As the light-absorbing medium is arranged betweenthe outside surface of the lamp vessel and the interference film, lightreflected by the interference film will pass the light-absorbing mediumtwice, which leads to a further improvement of the effectiveness of theabsorption process. In addition, light which is reflected to and frobetween the interference film on both sides of the lamp vessel passesthe light-absorbing layer twice at each reflection.

A thickness t_(abs) of the light-absorbing layer preferably lies in therange 5 nm≦t_(abs)≦5000 nm. If the thickness of the light-absorbinglayer is smaller than 5 nm, absorption hardly takes place and theintended shift of the color temperature is insufficiently achieved. Ifthe thickness of the layer exceeds 5 μm, too much light is absorbed,which adversely affects the lumen output of the electric lamp. Thedesired layer thickness is also prompted by the concentration of thepigment in the light-absorbing coating.

A preferred embodiment of the electric lamp is characterized in that thelight-absorbing medium has an amber-colored or red-colored transmission.Electric lamps which, in operation, emit amber-colored light canparticularly suitably be used as an indicator lamp in vehicles. Electriclamps which, in operation, emit red light are particularly suitable asbrake lights in vehicles.

The choice of selectively light-absorbing layers is limited by therequirement which, in accordance with the invention, is to be met by thesteepness of the change of the spectral transmission of thelight-absorbing medium. The choice of selectively light-absorbing layersis further limited by the thermal requirements to be met by such alight-absorbing layer. Said thermal requirements include the durabilityof the light-absorbing medium during the service life and the resistanceto changing temperatures of the lamp vessel.

Preferably, the light-absorbing medium has an amber-coloredtransmission. A particularly suitable light-absorbing medium ischromophtal yellow, chemical formula C₂₂H₆C₁₈N₄O₂ and C.I. (constitutionnumber) 56280. This organic dye is also referred to as “C.I.-110 yellowpigment”, “C.I. pigment yellow 137” orBis[4,5,6,7-tetrachloro-3-oxoisoindoline-1-ylidene)-1,4-phenylenediamine.An alternative light-absorbing medium having an amber-coloredtransmission is yellow anthraquinone, chemical formula C₃₇H₂₁N₅O₄ andC.I. 60645. This organic dye is also referred to as “Filester yellow2648A” or “Filester yellow RN”, chemical formula1,1′-[(6-phenyl-1,3,5-triazine-2,4diyl)diimino]bis-.

In an alternative embodiment, the light-absorbing medium has ared-colored transmission and comprises, by way of example, “chromophtalred A2B” with C.I. 65300. Said organic dye is alternatively referred toas “pigment red 177”, dianthraquinonyl red or as[1,1′-Bianthracene]-9,9′,10,10′-tetrone, 4,4′-diamino-(TSCA, DSL).

An embodiment of the electric lamp in accordance with the invention ischaracterized in that the interference film comprises layers, comprisinga first layer of a material having a comparatively low refractive indexand a second layer of a material having a comparatively high refractiveindex, said first and second layers preferably being stackedalternately. The use of two materials simplifies the provision of theinterference film. In an alternative embodiment, at least a third layerof a material is applied having a refractive index between that of thefirst layer and the second layer.

A preferred embodiment of the electric lamp in accordance with theinvention is characterized in that the first layer of the interferencefilm comprises predominantly silicon oxide, and in that the second layerpredominantly comprises a material whose refractive index is high incomparison with a refractive index of silicon oxide. Layers of siliconoxide can be provided comparatively readily using various depositiontechniques.

Preferably, the second layer of the interference film comprises amaterial selected from the group formed by titanium oxide, tantalumoxide, zirconium oxide, niobium oxide, hafnium oxide, silicon nitrideand combinations of said materials.

Preferably, the interference films are Nb₂O₅/SiO₂ type films, Ta₂O₅/SiO₂type films, TiO₂/SiO₂, ZrO₂/SiO₂ type films or mixtures thereof andcomprise, preferably, at least 5 and at most approximately 25 layers. Asa result of the comparatively small number of layers, the manufacturingcosts of such an interference film are comparatively low.

The light source of the electric lamp may be an incandescent body, forexample, in a halogen-containing gas, or it may be an electrode pair inan ionizable gas, for example, an inert gas with metal halides, possiblywith, for example, mercury as a buffer gas. The light source may besurrounded by an innermost gastight envelope. It is alternativelypossible that the outermost envelope surrounds the lamp vessel.

The interference film and the light-absorbing layer may be provided in acustomary manner by means of, for example, vapor deposition (PVD:physical vapor deposition) or by (dc) (reactive) sputtering or by meansof a dip-coating or spraying process or by means of LP-CVD (low-pressurechemical vapor deposition), PE-CVD (plasma-enhanced CVD) or PI-CVD(plasma impulse chemical vapor deposition). The light-absorbing layer onthe outer wall of the lamp vessel is preferably applied by means ofspraying. If the light-absorbing medium forms part of the wall of thelamp vessel, then this medium is generally provided in the wall in thecourse of the manufacture of the lamp vessel.

It has been found that the combination of absorbing medium andinterference film of the electric lamp in accordance with the inventionsubstantially retains its initial properties throughout the service lifeof the electric lamp.

These and further aspects of the invention will be apparent from andelucidated with reference to the embodiment(s) described hereinafter.

In the drawings:

FIG. 1A is a cross-sectional view of an embodiment of the electric lampin accordance with the invention;

FIG. 1B is a side view of an alternative embodiment of the electric lampin accordance with the invention;

FIG. 2A shows the reflection spectrum as a function of the wavelength ofa ZrO₂/SiO₂ interference film matching the spectral transition of anamber-colored light light-absorbing medium, and

FIG. 2B shows the reflection spectrum as a function of the wavelength ofa ZrO₂/SiO₂ interference film matching the spectral transition of ared-colored light light-absorbing medium.

The Figures are purely diagrammatic and not drawn true to scale. Somedimensions are particularly strongly exaggerated for reasons of clarity.Equivalent components have been given the same reference numerals asmuch as possible in the Figures.

FIG. 1A is a cross-sectional view of an embodiment of the electric lampin accordance with the invention. Said electric lamp has alight-transmitting lamp vessel 1, for example, of glass, which is sealedin a gastight manner and which accommodates an electric element 2, beingin the Figure a (spiral-shaped) tungsten incandescent body, which isconnected to current conductors 3 which issue from the lamp vessel 1 tothe exterior. The lamp shown, which is alternatively referred to asPY21W (12 volt, 21 watt), is filled with an inert gas, for example, anAr/N₂ mixture, having a filling pressure of approximately 1 bar.

In the embodiment of the electric lamp shown in FIG. 1A, thelight-absorbing medium is provided, in the form of a light-absorbingcoating 6, on an outside of the lamp vessel 1 (on a wall of the lampvessel), and an interference film 5 is provided on said light-absorbingcoating (also see FIG. 1B). The light-absorbing coating 6 comprises, inthis case, for example a layer of the pigment referred to as chromophtalyellow in a layer thickness of, for example, 2 μm. An electric lampprovided with such a light-absorbing medium emits, in operation,amber-colored light. Such electric lamps are used as an indicator lamp,for example, in indicators of vehicles, and their service life is atleast substantially 1200 hours. In an alternative embodiment of thecoating, the light-absorbing coating 6 comprises a layer of chromophtalred A2B having a layer thickness of for example 2 μm. An electric lampprovided with such a chromophtal red A2B layer emits, in operation,red-colored light. Such electric lamps are used as brake lights invehicles, and their service life is at least substantially 1200 hours.

In an alternative embodiment of the electric lamp shown in FIG. 1A, thewall of the lamp vessel comprises the light-absorbing medium.

FIG. 1B is a side view of an alternative embodiment of the electricincandescent lamp in accordance with the invention. Said electric lampcomprises a quartz glass lamp vessel 11 accommodating an incandescentbody as the light source 12. Current conductors 13 are connected to saidlight source and issue from the lamp vessel 11 to the exterior. The lampvessel 11 is filled with a halogen-containing gas, for example, hydrogenbromide. At least a part of the lamp vessel 11 is covered with alight-absorbing medium 16 in the form of a light-absorbing coatingwhich, in this example, is formed by (an MTMS matrix-containing)chromophtal yellow or chromophtal red A2B in a layer thickness ofapproximately 2 μm.

In FIG. 1A, an interference film 5 is applied to the light-absorbingmedium applied to the wall of the lamp vessel 1 (the “substrate”), whichinterference film comprises layers of alternately a layer of a materialhaving a comparatively high refractive index, for example titanium oxide(average refractive index of TiO₂ approximately 2.4-2.8), niobium oxide(average refractive index of Nb₂O₅ approximately 2.34), tantalum oxide(average refractive index of Ta₂O₅ approximately 2.18) or zirconiumoxide (average refractive index of ZrO₂ approximately 2.16), and a layerof, predominantly, silicon oxide (average refractive index approximately1.46). The TiO₂/SiO₂, Nb₂O₅/SiO₂, Ta₂O₅/SiO₂ or ZrO₂/SiO₂ interferencefilms preferably comprise only a small number of layers.

In the example shown in FIG. 1B, an interference film 15 is applied tothe light-absorbing medium 16 and comprises layers of, alternately,predominantly zirconium oxide and silicon oxide. The ZrO₂/SiO₂interference film preferably comprises only a small number of layers.

The lamp vessel 11 in FIG. 1B is mounted in an outer bulb 14, which issupported by a lamp cap 17 to which the current conductors 13 areelectrically connected. The lamp shown is a 60 W mains-voltage lamphaving a service life of at least substantially 2500 hours.

Experiments have shown that said interference films preferably compriseat least 5 and at most approximately 25 layers. For example, aninterference film having the required spectral characteristics for anamber-colored light absorbing layer (spectral transition foramber-colored light λ_(tr)˜550 nm) has a filter design comprising 18ZrO₂/SiO₂ layers as given in Table II.

TABLE II Design of an optical interference film matching the spectraltransition of an amber-colored light light-absorbing medium. Layer layerrefractive thickness number material index (nm)  1 SiO₂ 1.462 245.2  2ZrO₂ 2.066 128.3  3 SiO₂ 1.462 139.1  4 ZrO₂ 2.066 78.7  5 SiO₂ 1.462132.5  6 ZrO₂ 2.066 36.5  7 SiO₂ 1.462 109.9  8 ZrO₂ 2.066 91.3  9 SiO₂1.462 102.4 10 ZrO₂ 2.066 47.01 11 SiO₂ 1.462 68.2 12 ZrO₂ 2.066 54.5 13SiO₂ 1.462 95.6 14 ZrO₂ 2.066 34.1 15 SiO₂ 1.462 106.2 16 ZrO₂ 2.06637.7 17 SiO₂ 1.462 116.2 18 ZrO₂ 2.066 23.5 Substrate glass 1.521

FIG. 2A shows the calculated reflection spectrum (R) as a function ofthe wavelength (λ in nm) of a ZrO₂/SiO₂ interference film matching thespectral transition of an amber-colored light light-absorbing medium.The design of the interference film is according to Table II. It can beseen from FIG. 2A that in the wavelength range from 400 nm tillapproximately 550 nm (range where the amber-colored lightlight-absorbing medium is active) the reflection of the interferencefilm is approximately 0.75, whereas in the (visible) wavelength rangeabove approximately 550 nm (range where the amber-colored lightlight-absorbing medium is no longer active), the reflection of theinterference film is approximately 0.50 (ΔR≅0.25).

The interference film as depicted in FIG. 2A is close to color-neutralwith a gray appearance, the color coordinates being x=0.2770, y=0.3203and z=0.4027, whereas the color coordinates of the white point (for D65illumination) are x=0.3127, y=0.3291 and z=0.3582. An interference filmas depicted in FIG. 2A exhibits a reduced scatter level and the lumenloss in the range above approximately 550 nm (range where thelight-absorbing medium is no longer active) is reduced withapproximately 50%.

In an alternative embodiment, the interference film is designed to matchthe required spectral characteristics for a red-colored lightlight-absorbing layer (spectral transition for red-colored lightλ_(tr)˜600 nm). The filter has a design comprising 19 ZrO₂/SiO₂ layersas given in Table III.

TABLE III Design of an optical interference film matching the spectraltransition of a red-colored light light-absorbing medium. Layer layerrefractive thickness number material index (nm)  1 SiO₂ 1.462 138.8  2ZrO₂ 2.066 76.6  3 SiO₂ 1.462 61.9  4 ZrO₂ 2.066 39.5  5 SiO₂ 1.462 72.1 6 ZrO₂ 2.066 58.3  7 SiO₂ 1.462 83.3  8 ZrO₂ 2.066 57.3  9 SiO₂ 1.46283.9 10 ZrO₂ 2.066 82.6 11 SiO₂ 1.462 97.8 12 ZrO₂ 2.066 78.2 13 SiO₂1.462 69.0 14 ZrO₂ 2.066 91.0 15 SiO₂ 1.462 130.5 16 ZrO₂ 2.066 124.7 17SiO₂ 1.462 136.7 18 ZrO₂ 2.066 95.1 19 SiO₂ 1.462 142.4 Substrate glass1.521

FIG. 2B shows the calculated reflection spectrum (R) as a function ofthe wavelength (λ in nm) of a ZrO₂/SiO₂ interference film matching thespectral transition of a red-colored light light-absorbing medium. Thedesign of the interference film is according to Table III. It can beseen from FIG. 2B that in the wavelength range from 400 nm tillapproximately 600 nm (range where the red-colored light light-absorbingmedium is active) the reflection of the interference film isapproximately 0.75, whereas in the (visible) wavelength range aboveapproximately 600 nm (range where the red-colored light light-absorbingmedium is no longer active), the reflection of the interference film isapproximately 0.50 (ΔR≅0.25).

The interference film as depicted in FIG. 3A is close to color-neutralwith a gray appearance, the color coordinates being x=0.2968, y=0.3305and z=0.3726, whereas the color coordinates of the white point (for D65illumination) are x=0.3127, y=0.3291 and z=0.3582. An interference filmas depicted in FIG. 2B exhibits a reduced scatter level and the lumenloss in the range above approximately 550 nm (range where thelight-absorbing medium is no longer active) is reduced withapproximately 50%.

The interference film as depicted in Figure 2B is close to color-neutralwith a gray appearance, the color coordinates being x=0.2968,y=0.3305and z=0.3726, whereas the color coordinates of the white point(for DGS illumination) are x=0.3127, y=0.3291 and z=0.3582. Aninterference film as depicted in FIG. 2B exhibits a reduced scatterlevel and the lumen loss in the range above approximately 550 nm (rangewhere the light-absorbing medium is no longer active) is reduced withapproximately 50%.

It will be clear that, within the scope of the invention, many varationsare Possible to those skilled in the art.

The scope of protection of the invention os not limited to the examplesgiven Herin. The invention is embodied in each novel characteristic andeach combination of characteristics. References numerals in the claimsdo not limit the scope of protection thereof. the use of the verb “tocomprise” and its conjuctions does not exclude the presence of elementsother than those mentioned in the claims. The use of the article “a” or“an” in front of an element does not exclude the presence of a pluralityof such elements.

1. An electric lamp comprising a light-transmitting lamp vessel in whicha light source is arranged, said electric lamp comprising alight-absorbing medium exhibiting a spectral transition at a wavelengthλ_(tr) in the visible range, at least a part of the lamp vessel beingprovided with an interference film , wherein in a wavelength range0.95×λ_(tr)≦λ≦1.2×λ_(tr), a reflection R of the interference filmchanges from 0.60≦R≦0.95 to 0.40≦R≦0.65 with a step ΔR in a range0.2≦ΔR≦0.45, wherein the reflection R has a first reduced level and asecond reduced level beginning substantially from the wavelength λ_(tr),the first reduced level being substantially constant for approximately100nm more than the wavelength λ_(tr).
 2. An electric lamp as claimed inclaim 1, wherein in the wavelength range 0.95×λ_(tr)≦λ≦1.1×λ_(tr), thereflection R changes from 0.70≦R≦0.80 to 0.45≦R≦0.55 with a step ΔR inthe range 0.2≦ΔR≦0.3.
 3. An electric lamp as claimed in claim 1, whereina wall of the lamp vessel comprises the light-absorbing medium.
 4. Anelectric lamp as claimed in claim 1, wherein the light-absorbing medium(6; 16) comprises a light-absorbing coating which is situated betweenthe lamp vessel and the interference film.
 5. An electric lamp asclaimed in claim 4, wherein a thickness t_(abs) of the light-absorbinglayer lies in the range 5 nm≦t_(abs)≦5 μm.
 6. An electric lamp asclaimed in claim 1, wherein the electric lamp emits colored light, inoperation, and has an at least substantially color-neutral appearance inthe off state.
 7. The electric lamp as claimed in claim 1, characterizedin that the light-absorbing medium comprises an amber-colored orred-colored transmission.
 8. The electric lamp as claimed in claim 7,wherein the light-absorbing medium having an amber-colored transmissionis chromophtal yellow, chemical formula C₂₂H₆C₁₈N₄O₂ and C.I. 56280, orwherein the light-absorbing medium having an amber-colored transmissionis yellow anthraquinone, chemical formula C₃₇H₂₁N₅O₄ and C.I.
 60645. 9.An electric lamp as claimed in claim 8, wherein the light-absorbingmedium having a red-colored transmission is chromophtal red, chemicalformula C₂₈H₁₆N₂O₄ and C.I.
 65300. 10. The electric lamp as claimed inclaim 1, wherein the interference film comprises layers, comprising afirst layer of a material having a comparatively low refractive indexand a second layer of a material having a comparatively high refractiveindex, said first and second layers preferably being stackedalternately.
 11. The electric lamp as claimed in claim 10, wherein thefirst layer of the interference film comprises predominantly siliconoxide, and wherein the second layer predominantly comprises a materialwhose refractive index is high in comparison with a refractive index ofsilicon oxide.
 12. The electric lamp as claimed in claim 11, wherein thesecond layer of the interference film comprises a material selected fromthe group formed by titanium oxide, tantalum oxide, zirconium oxide,niobium oxide, hafnium oxide, silicon nitride and combinations of saidmaterials.
 13. A lamp comprising: a light source; an envelopesurrounding the light source; a light-absorbing medium formed over theenvelope for absorbing light having a wavelength greater than apredetermined wavelength; and an interference film formed over thelight-absorbing medium, wherein reflection R of the interference filmhas a first reduced level and a second reduced level lower than thefirst reduced level, the first reduced level beginning substantiallyfrom a wavelength λ_(tr), the first reduced level being substantiallyconstant for approximately 100nm more than the wavelength λ_(tr) and thesecond reduced level being substantially constant for a wavelength rangegreater than 50nm.
 14. The lamp of claim 13, wherein the first reducedlevel is reduced by 0.2 to 0.45.
 15. The lamp of claim 13, wherein in awavelength range 0.95×λ_(tr)≦λ≦1.2×λ_(tr), the reflection R changes from0.6≦R≦0.95 to 0.40≦R≦0.65 with a step ΔR in a range 0.2≦ΔR≦0.45.
 16. Thelamp of claim 13, wherein in a wavelength range0.95×λ_(tr)≦λ≦1.1×λ_(tr), the reflection R changes from 0.70≦R≦0.80 to0.45≦R≦0.55 with a step ΔR in the range 0.2≦ΔR≦0.3.
 17. The lamp ofclaim 13, wherein a thickness t_(abs) of the light-absorbing layer liesin the range 5 nm≦t_(abs)≦5 μm.
 18. The lamp of claim 13, wherein thelamp emits colored light in operation, and has an at least substantiallycolor-neutral appearance in an off state.
 19. A lamp comprising: a lightsource; an envelope surrounding the light source; a light-absorbingmedium formed over the envelope for absorbing light having a wavelengthgreater than a predetermined wavelength; and an interference film formedover the light-absorbing medium, wherein reflection R of theinterference film has a first reduced level and a second reduced levellower than the first reduced level, the first reduced level beginningsubstantially from a wavelength λ_(tr), the first reduced level beingsubstantially constant for approximately 100nm more than the wavelengthλ_(tr) and the second reduced level being substantially constant for awavelength range, wherein in a wavelength range 0.95×λtr≦λ≦1.2×λtr, thereflection R changes from 0.60≦R≦0.95 to 0.40≦R≦0.65with a step ΔR in arange 0.2≦ΔR≦0.45.
 20. A lamp comprising: a lignt source; an envelopesurrounding the light source; a light-absorbing medium formed over theenvelope for absorbing light having a wavelength greater than apredetermined wavelength; and an interference film formed over thelight-absorbing medium, wherein reflection R of the interference filmhas a first reduced level and a second reduced level lower than thefirst reduced level, the first reduced level beginning substantiallyfrom a wavelength λ_(tr), the first reduced level being substantiallyconstant for approximately 100nm more than the wavelength λ_(tr)and thesecond reduced level being substantially constant for a wavelengthrange, wherein in a wavelength range 0.95×λ_(tr)≦λ≦1.1×λ_(tr), thereflection R changes from 0.70≦R≦0.80 to 0.45≦R≦0.55 with a step ΔR inthe range 0.2≦ΔR≦0.3.